This invention relates generally to exhaust systems and methods, and more particularly to an energy efficient and environmentally sound advanced stack system and method for exhausting toxic air from fume hoods.
Laboratories and other facilities typically contain fume hoods in which chemical processes produce toxic fumes. These facilities necessarily contain fume exhaust stack systems that exhaust this toxic air from the building and send the toxic air through a stack to a prescribed minimum altitude such that fresh air contamination and environmental pollution is reduced. To satisfy environmental safety standards, the fume exhaust stack system must provide a minimum velocity and momentum to the toxic exhaust exiting the stack to ensure that the toxic exhaust reaches a minimum altitude substantially higher than the outlet of the stack. Due to architecture, structural, and economic limitations, however, the stack is often required to be as short as possible.
A typical fume hood exhaust stack system (depicted in FIG. 1) consists of a stack, a constant-speed fan, a make-up air damper, fume hoods, a static pressure sensor, and a controller. In these conventional fume exhaust stack systems, the total toxic air flow rate and volume from the fume hoods are significantly below the optimal design values because the majority of the fume hoods are typically in a standby mode in which they exhaust little or no toxic air into the exhaust header. These system utilize a controller that modulates a make-up air damper to maintain a static pressure set point at the exhaust header. When the toxic air flow rate decreases to a value less than the design value, the controller opens the make-up air damper to maintain the desired static pressure set point; conversely, when the toxic air flow rate increases, the controller closes the make-up air damper to maintain the set point. Since the static pressure sensor is often located far from the mixing junction of the make-up air conduit and exhaust header discharge conduit, the static pressure at the inlet of the fan is significantly higher under partial exhaust air flow conditions than under full exhaust air flow conditions.
Under partial exhaust conditions—when the make-up air damper is open or partially open—the air flow rate through the fan is higher than the design value due to the higher static pressure at the inlet of the fan. The fan power consumption under these partial-exhaust high static pressure conditions is often up to 30% greater than the fan power consumed under full exhaust conditions; fan power consumption increases as exhaust air flow rate decreases. Fan power consumption increases as exhaust air flow decreases. The exhaust fans generally operate for 8,760 hours annually, while the fume hoods generally operate less than two hours per day. These existing systems therefore consume excess power and overload the fan motor when the exhausted toxic air flow from the fume hoods is less than the design value.
There is therefore a need for an energy efficient fume hood exhaust stack system and method that reduces fan power consumption while ensuring that the toxic exhaust discharged from the stack exits the stack with a relatively constant velocity and momentum sufficient to ensure that the toxic exhaust reaches an environmentally sound elevation.